Rivets of aluminum or the like are well known in the fastening arts and are widely used for securing together overlapping, relatively thin section structural members such as sheet metal. For example, in the aircraft industry it is a well known and accepted practice to use aluminum rivets to secure the skin of an aircraft to the airframe. Such rivets offer a high strength to weight ratio and therefore provide sufficient strength for the purpose with mimimum weight contribution to the aircraft.
For this and numerous other applications the rivet type most commonly utilized is referred to as a blind setting rivet as the rivet is installed entirely from the front side of the joint (i.e. the side from which the rivet is inserted before setting), without any need for access to the opposite side of the joint as is required for the installation of hot rivets in structural steel work, for example. Perhaps the most common type of blind rivet is the so-called "pop rivet" which is widely available as a consumer product.
Typically a blind rivet is formed as a hollow elongated sleeve of aluminum having an enlarged head with an axial through bore aligned with the hollow interior of the sleeve. The rivet is provided with a sacrificial rivet setting device that includes a mandrel positioned adjacent the end of the rivet sleeve opposite the head and an elongated shank which extends from the mandrel through the hollow interior of the rivet sleeve and outwardly of the head. As is well known, for installation such a rivet is positioned in a predrilled hole in the materials to be joined and tension is applied to the mandrel shank to pull the mandrel toward the rivet head and thereby deform that portion of the rivet sleeve which protrudes through the hole in which the rivet is to be set. As a result the protruding portion of the rivet sleeve is upset, radially expanded, or otherwise deformed to set the rivet. A breakoff section of the mandrel shank, such as a reduced diameter section for example, fractures with the application of further tension and/or bending after the rivet is fully set, thus freeing the mandrel and the shank from the rivet and leaving only the set rivet in place. This provides a strong permanent fastener of minimum weight.
The concept of self-drilling blind rivets is well known but the co-inventors herein are not aware of any which have been commercially successful; the major reason is excessive cost. The self-drilling rivet in its various forms is intended to simplify and speed up rivet installation by providing a drilling tip on the rivet structure whereby the hole to receive the rivet may be drilled, and the rivet then inserted and set as above described in a continuous operation with a single tool.
Previous self-drilling rivet concepts typically have included a tip portion of the sleeve opposite the rivet head, or a protruding portion of the rivet setting mandrel, which is formed as a drill bit for drilling the required hole. Examples of prior art self-drilling blind rivets include the following: U.S. Pat. Nos. 3,935,786; 3,772,957; and 3,750,518, all disclosing self-drilling rivets wherein the drill bit is formed as the forwardmost end of the rivet setting mandrel. U.S. Pat. No. 4,293,258 discloses a self-drilling blind rivet wherein the drill bit is a formed end of the rivet sleeve. British Patent Specification No. 1,484,260 discloses another self-drilling blind rivet.
Prior art self-drilling rivets generally have been subject to certain shortcomings. For example, the production of a one-use only drill bit on the tip of a rivet mandrel or rivet sleeve adds to the cost of the rivet and this incremental cost element reduces the cost benefit available from the attendent productivity improvements. Furthermore, to be effective for drilling the rivet hole, the drilling tip preferably is to be made of harder material than the rivet itself. The rivet must be readily deformable in setting thereof without deformation of the material surrounding the hole in which it is set, but the drill tip must be hard enough to readily drill the hole through the same material. In design practice, this problem typically imposes a weight and/or cost penalty. For example, in applications where the drilling tip remains as part of the installed rivet in the finished structure after rivet setting, the added weight of a steel drill tip would be undesirable; additionally, the aluminum-to-steel contact could cause long-term corrosion problems. If the drill tip is instead a lighter (i.e. lower density) material, a cost penalty typically arises.